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Karteikarten von sophietevans, aktualisiert more than 1 year ago
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| Frage | Antworten |
| How many domains do ligand-gated ion channels frequently consist of? | Four domains, which are usually organised as one large protein or an oligomeric array of separate subunits. |
| How many membrane-spanning helices does each subunit of a ligand-gated ion channel typically consist of? | 2-6, which form a cluster surrounding a central transmembrane pore. |
| What makes a ligand-gated ion channel 'closed'? | The transmembrane helices that compose the subunits of the channel are sharply 'kinked' inwards, forming a constriction when the channel is closed which prevents ion passage. These are moved out of the way when the appropriate ligand binds to the appropriate subunit, e.g. acetylcholine to the α-subunit of the cholinergic nicotinic receptor, causing the subunit to change conformation. |
| How do transmembrane helices making up the subunits of ligand-gated ion channels determine the selectivity of the ion channel? | If the amino acids composing them consist of primarily negatively charged amino acids, the ion channel will repel anions and be cation-selective. Of the cations themselves, the channels may be relatively non-selective, depending on it size, and the binding of a specific ligand to its ligand-binding site may induce a general cation influx which by proxy increases the concentration of Na+ ions in the cell causing depolarisation, for example. |
| What is the first step in signal transduction involving cAMP generation? | A ligand binding to its membrane receptor e.g. salbutamol binding to its β2-adrenoceptor, which causes the receptor to undergo a conformational change. |
| After a ligand has bound to its appropriate G-protein coupled receptor, there is a conformational change to the receptor. What does this do? | This exposes a new binding site on its inner surface, to which a G protein is able to bind. |
| What happens to the G protein when it is able to bind to the receptor? | In its resting state, the G protein has guanosine diphosphate (GDP) bound to it, but in binding to the receptor ligand complex, the G protein changes conformation and its guanyl nucleotide binding site changes shape. This weakens the intermolecular bonding forces between the binding site and GDP, and GDP is released. The binding site is now the correct conformation for guanosine triphosphate (GTP), which binds. |
| What happens to the G protein when GTP has bound to it? | The G protein undergoes another conformational change (the first being when it bound to the ligand-receptor complex and its affinity for GDP decreased while that for GTP increased), resulting in weakening in the links between the protein subunits that the α-subunit (with GTP attached) dissociates from the βγ subunit. |
| What determines whether the binding of the ligand to the receptor will have stimulately or inhibitory intracellular signalling effects? | The alpha-subunit has enzymatic properties, the nature of which determine whether the subsequent intracellular signalling leads to a stimulatory or inhibitory response, and which determines which protein is involved e.g. adenylate cyclase or phospholipase C. |
| What effect does the alpha-s subunit have? | It stimulates adenylate cyclase, increasing cAMP formation. |
| What effect does the alpha-i subunit have? | It inhibits adenylate cyclase, decreasing cAMP formation. It may also activate K+ ion channels. |
| What effect does the alpha-o subunit have? | It activates receptors that inhibit neuronal Ca2+ channels. |
| What effect does the alpha-q subunit have? | It activates phospholipase C, increasing IP3 and DAG formation. |
| What does adenylate cyclase do? | It catalyses the production of the nucleotide cyclic adenosine monophosphate (cAMP) from ATP for as long as the alpha-s subunit is bound to it, producing several hundred cAMP molecules and significantly amplifying the signal which started with 1 ligand bound to 1 receptor. |
| Why is activation of adenylate cyclase by the alpha-s subunit self-limiting? | Because alpha-s binding to adenylate cyclase increases its GTPase activity (magnitude according to specific effector), resulting in it hydrolysing its GTP to GDP + Pi, terminating its activity as it undergoes a conformational change, migrates back to the beta-gamma subunit, and reforms in its resting state. |
| What are the possible intracellular effects that result from cAMP production? | cAMP activates protein kinase A, which: phosphorylates and therefore activates myosin light chain kinase, resulting in inhibition of smooth muscle contraction e.g. the bronchodilating effect of salbutamol; phosphorylates and therefore activates lipase to increase lipolysis; phosphorylates and therefore activates phosphorylase kinase which results in increased glycogen catabolism; and phosphorylates and therefore inactivates glycogen synthase to reduce glycogen synthesis. |
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